Echo-type equalizer which differentiates echo signals



Dec 6, 1966 KolcHl ENDO ETAL 3,290,607

ECHO-TYPE EQUALIZER WHICH DIFFERENTIATES ECHO SIGNALS 2 Sheets-Sheet l Filed April 22, 1963 .r m v ombmobm IZOZV .Emo-

All u United States Patent i ECHQ-TYPE EQUALIZER WHICH DIFFEREN- TIATES ECHO SIGNALS Koichi Endo, Tokyo, and Masao Kawashima, Yokohamashi, Japan, assignors to Fujitsu Limited, Kawasaki, Japan, a corporation of Japan Filed Apr. 22, 1963, Ser. No. 274,764 6 Claims. (Cl. 328-162) Our invention relates to equalizers, particularly for restoring the wave shape of broad-band wave-forms, such as television video signals or pulse signals, which have been distorted by transmission.

Broad-band wave-form signals such as of television video signals or pulse signals tend to become :distorted by transmission lines which fail to exhibit fiat frequency characteristics with regard to amplitude or group delay time. str-ucted with passive networks are effective for eliminating such amplitude distortion and group delay distortion. However, amplitude equalizers usually produce group de- -lay distortion while group delay equalizers, unless their elements are lossless, produce amplitude distortion, thus rendering it difficult to completely suppress both types of distortion. Moreover, transmission lines frequently cause wave distortions unrelated to amplitude and group delay which cannot Abe equalized entirely by the above-mentioned conventional equalizers.

Known for compensating such distortions are time equalizers or echo equalizers. In such devices, a waveform to be equalized is applied to the input end of .a delay circuit whose other end constitutes a non-reflective terminal resistance. A main signal is extracted from one tap of the delay circuit while echo signals are extracted at integral multiples of the time T before and after the main signal. The echo signals are adjusted, combined with each other and then with the main signal, to produce a comparatively equalized output. However, such equalizers have outputs requiring still further correction.

It is an object of our invention to provide an echo-type equalizer which serves to reconstitute or equalize transmitted signals in a more reliable manner, which will avoid the above-mentioned deficiencies of known echo equal- 12ers.

To this end, and according to a feature of our invention, in an echo equalizer of the above type, we differentiate the combined signals from the adjusted echo taps before combining them with the output at the m-ain tap.

The invention, its other objects and advantages, will be explained more fully in the following deta-iled description `of an echo equalizer when read in the light of the accompanying drawings, wherein:

FIG. 1 is a schematic representation of an echo equalizer embodying features of the present invention;

FIG. 2 is a gain-frequency graph showing the response of the differentiating circuit in FIG. 1;

FIG. 3 is a time-voltage graph of an ideal disto-rtionless wave-form;

FIG. 4 `is a time-voltage graph corresponding to that of FIG. 3, but showing a distorted wave-form FIGS. 5a and 5b are time-voltage graphs of time-voltage curves illustrating the equalization corresponding to known echo-type equalizers; and

FIGS. 6a and 6b are time-voltage graphs illustrating the effects of wave-forms in the equalizer according to the invention.

In FIG. 1 a wave-form to be equalized (from a transmission lineor the like) is .applied to input terminals Inp of a delay line D whose other end is connected to a non-refiective terminal resistance R. The delay line is comprised of a plurality yof' bridge capacitors.

Known amplitude and group delay equalizers con- A main` rice signal is extracted from the delay line at the terminals a0 .and directed to an output Out by means of an amplifier AD. A number of echo signals are extracted from the delay line D at `the point a0 and other points in the line D which are placed before and after the main signal taps a0 at intervals of To, the other points being identified as am a', before the taps a0, and al an after a0. Extracting these respective signals are .a plurality of elements A'm Al, A0, A1, An, each forming respective interlocked differential capacitors of which one pair of terminals is connected parallel to the bridge circuit. The differential capacitors are connected as shown so that their respective capacitances measured from the side of the delay Icircuit remain constant even though they are varied, but that their output sides vary as their plates are moved.

The opposite sides of the outputs in the adjustment circuits A', Alfzm, 2,1; p=0,1, n) are combined two grou-ps each of which is applied to the respective grid inputs of respective vacuum tubes V1 and V2, forming a differential amplifier AMP. The vacuum tubes V1, V2 possess respective cathodes which are connected to ground by a common resistor. The plate of vacuum tube V1 is connected to a power supply source B+. The load for the amplifier AMP constitutes a differentiating or LCR circiut DIF tuned to a frequency fc.

The differentiating circuit DIF constitutes an LCR tuning circuit lhaving a tuning frequency fc near the cut-off point of the system and an arbitrary Q such that the slope of the amplitude-frequency characteristic is almost linear in the lower side band of the tuning frequency. The response of the differentiating circuit DIF is shown in FIG. 2. The output of the differentiating circuit passes by means of a coupling capacitor to the summing amplifier AD comprised of vacuum tubes V3 and V4 and forming a linear sum of the differentiated echo signals and the main signals. The equalized signal is extracted from the common load L of the summing amplifier AD.

An ideal non-distorted wave-form which may have been transmitted at a transmiter is illustrate-d in FIG. 3, showing voltage relative to time or V/ V0 relative to l/T. A transmission line will distort such a wave-form to the shape shown in FIG. 4. When this is applied to the inputs of delay line D it is delayed and extracted at the terminals r1.0 and applied to the grid of tube V4. Remaining echoes `which are delayed respectively for less and longer times than the main signal are tapped off at respective terminals a', and ap and adjusted with capacitor `adj-usters Ai and AP constituting differential condensers having two Ioutputs each. These two outputs of the differential condensers are combined or added respectively into two groups which are supplied to the grids of tubes V1 and V2 forming the differential amplifier AMP. The output of the differential amplifier AMP is shown in the voltage-time 4graph as curve A of FIG. 5b.

This is applied to the load DIF of the amplifier, an LCR tuning circuit `or differentiating circuit having a tuning frequency near the cut-off point of the system and a Q such that the slope of the amplitude-frequency characteristic is almost linear in the lower-side band Of the tuning frequency. The output of the differentiated circuit of the echo signal is shown in the voltage-time graph of FIG. 6b. The signal passes to the summing amplifier AD to produce an output corresponding to FIG. 6a. The output is taken across the common load L of amplifier AD.

The above may be lbetter understood from a consideration of the following theoretical matters.

In the delay line, the main signal is extracted from the taps a0; the signals advancing before the main signal and placed at 4time intervals To are extracted through the adjusting elements Al to Am at the respective voltages Kle-J'WTU to Kme-jmwTo. The signals lagging behind the main signal and placed at intervals of T are extracted through the adjusting elements A1 to An lat the respective voltages Klew'o to Klenw'o (m=M and n=N; w.=the greek lower case omega). In known devices these corrective echo wave-forms are extracted and added directly to the main signal at the auxiliary amplifier AMP, equalizying it, and are picked up at the output terminal Out. This corresponds to a device such as that shown in FIG. l, only substituting a resistive load for circuit DIF.

In such known equalizers the transmission characteristices in the range between the input and output terminals can be given mathematically as Hence the amplitude characteristics and the group delay characteristics (d/dw of the phase characteristics) of the time equalizer are respectively expressed as N Z kn cos (nwTO) n=M+1 N Z kn cos (nwTO) n=M-I-1 If M=N, then Thus, this time equalizer can be characterized as the cosine type. Each term forming the summation in A=(w) is a cos-term with a coetiicient of (km-l-km). Each cos (mwTo) term of the above equation is weighted byv (kn1-}-km) because the function cos X where X=mwT0 exhibits .a maximum value at X=O. Thus, in the case of the cosine-type equalization, unnecessary or excessive equalization occurs in the vicinity of a direct current component (hereinafter referred to ias the DC. component) where w=0, in all terms. When humps are equalized at a number of points on the frequency scale, such unnecessary equalizations are accumulated in the v-icinity of the D.C. component. To counterbalance this, further correction is required, which must also be protected from excessive equalization constituting distortion. As a result, an extremely complicated repetitive adjustment of the equalizer is necessary.

FIG. 3 illustrates an example of ideal distortionless step waveform expressed mathematically as A transmission line distorts this waveform to a form such as shown in FIG. 4. The distortion is at its maximum at the time point where it requires correction. FIG. 5a shows the corresponding equalized waveforms. The upper waveform represented in a dotted yline is the one equalized by the echo A shown in FIG. 5b. The equalization in this case is applied to a large extent to the low-frequency cornponents of the wave, with the result that while equalization is effected at the desired time points, unwanted distortion occurs at the other later time domain. When the composite wave of the echos A and B', FIG. 5b, is used for correction, the equalization of the low-frequency components can be reduced considerably. However, wave correction at the desired point is still incomplete, as shown by the lower waveform in FIG. 5a, because of the broadness of the composite wave.

By virtue of differentiator DIF, the present invention eliminates the above drawback to the conventional cosinetype time equalizer while nevertheless achieving a simple and reliable time equalizer, Corrective echoes are added to the main signal after passing an :auxiliary amplifier and a differential circuit. If the auxiliary amplifier is disregarded for simplicity of representation, the transmission characteristics of the equalizer in FIG. 1 'between its input terminal (Inp) and output terminal (Out) are as follows, inasmuch as the differentiated output of each tap is added to the main signal:

im sin (nT0w)] N cm) sin (mTow) -l- E Assuming that M=N, for simplicity of representation, then nlm sin (nTowiI Thus, in the frequency domain, za cosine term is included in the formulation of the group delay time but only sine terms are included in the equation showing amplitude characteristics which represent energy. In the group delay time, the sine series of the second groups of the terms in the equation, having the coeflicient w. whose magnitude increases in proportion to frequency, is greater in the higher frequency range. Thus, in thelong run, approximately constant equalization characteristics of sine type occur around weO, namely in the lower frequency range. The above, in the frequency domain, means that unnecessary or ex-cess equalization does not occur in and around the D.C. component. This will be further clarified by the following explanation of waveform equalization in the waveform domain.

FIG. 6a shows the waveform into which the distorted one in FIG. 4 is equalized, while HG. 6b illustrates an associated corrective echo. Since this echo is differentiated, it is expressed as The width of the echo-waveform is narrow and the Waveform possesses orthogonality at the sampling point. That is, judging from the above lformation regarding the frequency domain, no unnecessary corrective echo-Wave- :form in the time domain occurs in the vicinity of the D C. component. As a result, only the porti-on of the distorted waveform requiring equalization is equalized.

We claim:

1. An echo equalizer comprising delay means having a main tap and a plurality of echo taps, respective adjuster means connected to each echo tap and having respective outputs, said adjuster means providing signals at their outputs, circuit means connected to the outputs of the adjuster means for adding their signals, and differentiating means connecting the output of said circuit means to said main tap for diiferentiating the eche signals and producing With the signal at the main tap an equalized output signal.

2. An echo equalizer comprising delay means having a main tap and a plurality of echo taps, respective adjuster means connected to each echo tap and having respective outputs, said adjuster means providing signals at their outputs, circuit means connected to the outputs of the adjuster means for adding their sign-als, and differentiating means connecting the output of said circuit means te said main tap for diiferentiating the echo signals and producing with the signal at the main tap an equalized output signal, said diiferentiating means including a tuning circuit.

3. An echo equalizer comprising delay means having a main tap and a plurality of echo taps, respective adjuster means connected to each echo tap and having respective outputs, .said adjuster means providing signals at their outputs, circuit means connected to Jthe outputs of the adjust-or means for adding their signals, Iand differentiating means connecting the output of said circuit means to said main tap for dilferentiating the echo signals and producing with the signal at the main tap an equalized output signal, said differentiating means including a capacitor, an inductor and resistor in parallel connection.

4. An echo equalizer comprising delay means having a main tap and a plurality ott echo taps, respective adjuster means connected to each echo tap and having respective outputs, said adjustor means providing signals at their outputs, circuit means connected to the outputs of the adjuster means for adding their signals, and differentiating means connecting the output of said circuit means to said main tap for diiferentiating the echo signals and producing with the signal at the main tap an equalized output signal, said differentiating means including a capacitor, an inductor and resistor in parallel connection, said circuit means including an amplifier, said diiferentiating means forming the load for said amplifier.

5. An echo equalizer comprising a delay line, a plurality of taps forming a main signal output at one location along the line and a plurality of echo signal outputs at earlier and later locations along the line, a resistor at the end of the line to absorb reflections, respective capacitor adjuster means connected to each of said outputs and having each a pair of output terminals, .a differential amplifier having one input terminal connected to the rst output terminal on each yadjuster means and a second input terminal connected to the other output terminal of each adjuster means, a differentiating circuit lforming a load for said differential amplier, `and amplifier means connecting the voltage of said dilferentiating lcircuit to the main output to form an equalized signal.

6. An echo equalizer comprising a delay line, a plurality o taps forming a main signal output at one location along the -line and a plurality of echo signal -outputs at earlier and later locations along the line, a resistor at the end of the line to absorb reections, respective capacitor adjuster means connected to each of said outputs and having each a pair -of output terminals, a diiferential amplier having one input terminal connected to the rst output terminal on each adjuster means and a second input terminal connected to the other output terminal of each adjuster means, a differentiating circuit forming a load for said differential amplier, and amplier means connecting the voltage of said ditferentiating circuit to the main output to form an equalized signal, said differentiating circuit comprising a tank circuit having a capacitor inductor and resistor.

References Cited by the Examiner UNITED STATES PATENTS 2,790,956 4/ 1957 Ketchledge 333-29 2,975,371 3/196'1 Greanas 333-39 2,976,516 3/1961 Taber 333-39 3,092,809 6/1963 Merritt et al 333-29 3,181,089 4/ 1965 Fujimoto 333-29 ARTHUR GAUSS, Primary Examiner. R. H- EPSTEIN, Assam Examiner, 

1. AN ECHO EQUALIZER COMPRISING DELAY MEANS HAVING A MAIN TAP AND A PLURALITY OF ECHO TAPS, RESPECTIVE AJUSTOR MEANS CONNECTED TO EACH ECHO TAP AND HAVING RESPECTIVE OUTPUTS, SAID ADJUSTOR MEANS PROVIDING SIGNALS AT THEIR OUTPUTS, CIRCUIT MEANS CONNECTED TO THE OUTPUTS OF THE ADJUSTOR MEANS FOR ADDING THEIR SIGNALS, AND DIFFERENTIATING 